At high frequencies, resonant circuits such as filter circuits, use distributed components in place of the discrete lumped circuit components used at lower frequencies. Hence, distributed component filter circuitry is used in microwave radio circuits as the band pass and band reject filters in the various stages of the radio circuitry. These filters are typically required, depending on their immediate application, to meet certain size, weight and performance constraints.
Coupled line filter structures having interdigital or comb line structures are frequently used for these applications. These filters include a series of resonant elements arranged in a comb like pattern in an air chamber. These resonant elements are tuned so that the coupled line exhibits some desired frequency characteristic such as frequency band pass. They are easy to manufacture since they do not demand tight tolerances, however for many applications their size and weight may be too great and/or their electrical characteristics such as Q (quality) may not provide the desired electrical performance.
For a given frequency and performance response, a standard resonant cavity filter may have a size exceeding permissible dimensions required by the circuitry package for the intended application. It may have electrical performance requirements that require a high Q value not attainable in the structure under consideration.